Particle Toughening for Improving Fracture Toughness

ABSTRACT

A curable prepreg ply formed by applying two outer resin films to the top and bottom surfaces, respectively, of a layer of resin-impregnated reinforcement fibers. The outer resin films contains insoluble toughening particles, and partially soluble or swellable toughening particles, but the resin matrix which impregnates the reinforcement fibers does not contain the same toughening particles. The insoluble toughening particles are insoluble in the resin matrix of the resin films upon curing of the prepreg ply. The partially soluble or swellabbe toughening particles are partially soluble or swellable in the resin matrix of the resin films upon curing of the prepreg ply, but remain as discreet particles after curing.

BACKGROUND

Fiber-reinforced polymer (FRP) composites have been used ashigh-strength, low-weight engineering materials to replace metals inaerospace structures such as primary structures of aircrafts. Importantproperties of such composite materials are high strength, stiffness andreduced weight.

Multiple layers of prepreg plies are commonly used to form structuralcomposite parts that have a laminated structure. Delamination of suchcomposite parts is an important failure mode. Delamination occurs whentwo layers de-bond from each other. Important design limiting factorsinclude both the energy needed to initiate a delamination and the energyneeded to propagate it.

A cured composite (e.g. prepreg layup) with improved resistance todelamination is one with improved Compression Strength After Impact(CAI) and fracture toughness (G_(Ic) and G_(IIc)).

CAI measures the ability of a composite material to tolerate damage. Inthe test to measure CAI, the composite material is subject to an impactof a given energy and then loaded in compression. Damage area and dentdepth are measured following the impact and prior to the compressiontest. During this test, the composite material is constrained to ensurethat no elastic instability is taking place and the strength of thecomposite material is recorded.

Fracture toughness is a property which describes the ability of amaterial containing a crack to resist fracture, and is one of the mostimportant properties of a material for aerospace applications. Fracturetoughness is a quantitative way of expressing a material's resistance tobrittle fracture when a crack is present.

Fracture toughness may be quantified as strain energy release rate(G_(c)), which is the energy dissipated during fracture per unit ofnewly created fracture surface area. G_(c) includes G_(Ic) (Mode1—opening mode) or G_(IIc) (Mode II—in plane shear). The subscript “Ic”denotes Mode I crack opening, which is formed under a normal tensilestress perpendicular to the crack, and the subscript “Ilc” denotes ModeII crack produced by a shear stress acting parallel to the plane of thecrack and perpendicular to the crack front. The initiation and growth ofa delamination is often determined by examining Mode I and Mode IIfracture toughness.

SUMMARY

Disclosed herein is a curable prepreg ply having two resin films appliedto the top surface and the bottom surface, respectively, of a layer ofresin-impregnated reinforcement fibers, wherein the resin films containsinsoluble toughening particles, and partially soluble or swellabletoughening particles, but the resin matrix which impregnates thereinforcement fibers does not contain the same toughening particles. Theinsoluble toughening particles are insoluble in the resin matrix of theresin films upon curing of the prepreg ply. The partially soluble orswellabbe toughening particles are partially soluble or swellable in theresin matrix of the resin films upon curing of the prepreg ply, butremain as discreet particles after curing. A composite structure may beformed by laying up a plurality of these prepreg plies.

Methods relating to fabricating the prepreg ply and the compositestructure are also disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D illustrates a four-film process for making a prepreg ply.

FIG. 2 illustrates an exemplary system for carrying out the four-filmprocess, according to one embodiment.

FIG. 3A illustrates an exemplary prepreg forming system according toanother embodiment, wherein a resin bead is formed ahead of the firstpressure nip.

FIG. 3B is an exploded view of the resin bead shown in FIG. 3A.

FIG. 4 is an optical microscopy image showing a cross-sectional view ofa cured laminate formed according to a two-film process.

FIG. 5 is an optical microscopy image showing a cross-sectional view ofa cured laminate formed according to a four-film process.

FIG. 6A is a magnified view of the interlaminar regions of the curedlaminate shown in FIG. 4.

FIG. 6B is a magnified view of the interlaminar regions of the curedlaminate shown in FIG. 5.

DETAILED DESCRIPTION

Attempts have been made to toughen the interlaminar region betweenadjacent prepreg plies using toughening particles. Un-crosslinked,soluble thermoplastic particles have been used to toughen thermosettingresin systems but they have been associated with various problems. Oneproblem with thermoplastic particles that dissolve during curing is thatthe resulting composite does not retain sufficient thermosettingthermomechanical properties. Some insoluble particles do not allow theresin material to penetrate the particles causing a de-bonding betweenthe particles and the resin matrix, thus, they do not confer sufficientstrength to the composite material. Therefore, the selection oftoughening particles is important.

It has been found that by incorporating certain blend of tougheningparticles into the interlaminar regions of a multilayer composite, theCAI and fracture toughness of the final, cured composite may beimproved. The multilayer composite in this context refers to a laminatecomposed of multiple structural layers arranged in a stackingarrangement (i.e. a lay-up or a laminate). Each structural layer iscomposed of resin-impregnated fibers, i.e. reinforcing fibersimpregnated with a resin matrix. The “interlaminar region” refers to theregion between two adjacent structural layers of reinforcement fibers.

Moreover, it has been discovered that the G_(IIc) fracture toughness ofthe multilayer composite may be further improved by incorporating aspecific blend of insoluble toughening particles and partially soluble(or swellable) toughening particles into the interlaminar regions of amultilayer composite in addition to utilizing a four-film process toapply the toughening particles to the structural layers. It was foundunexpectedly that the placement of the toughening particles via thefour-film process results in a substantially uniform interlaminar regionbecause the particles do not migrate away from the interlaminar regionupon curing of the multilayer composite (or prepreg layup).

This improvement is seen when compared to the same composite in whichthe blend of toughening particles is applied via a two-film processduring manufacturing. However, the same improvement in G_(IIc) fracturetoughness is not seen when the four-film process is utilized but onlyone type of toughening particles (either insoluble or partiallysoluble/swellable) is incorporated into the interlaminar regions.

FIGS. 1A-1D illustrates the four-film process mentioned above. Referringto FIG. 1A, two resin films 11, 12 are applied to the top and bottomsurfaces, respectively, of a layer of reinforcement fibers 10. Thefibrous reinforcement 10 may be unidirectionally aligned fibers (i.e.continuous fibers aligned on the same plane and in the same direction).However, it should be understood that the reinforcement fibers 10 may bealigned in multiple directions or may take the form of a woven fabric.Heat and pressure are then applied to the resulting assembly to form aresin impregnated fiber layer 13 as shown in FIG. 1B. Referring to FIGS.1C and 1D, two additional resin films 14, 15 are subsequently pressedonto the top and bottom surfaces, respectively, of the resin-impregnatedlayer 13 to yield a composite layer 16, also referred to as a “prepreg”or “prepreg ply”. To form a composite structure, a plurality ofcomposite layer 16 are laid up in a stacking arrangement to form acomposite lay-up having toughening particles in the interlaminar regionbetween adjacent composite layers.

In one embodiment, the resin films 11, 12, 14, 15 are formed fromsubstantially the same curable thermoset resin matrix with thedifference being that the outer films 14 and 15 contain a mixture ofinsoluble and partially soluble or swellable toughening particles,whereas the first two films 11 and 12 do not. The ratio of (i) insolubletoughening particles to (ii) partially soluble or swellable tougheningparticles in the resin matrix forming the outer resin films 14, 15 maybe ranging from 20:80 to 80:20.

FIG. 2 illustrates an exemplary system for carrying out the four-filmprocess. A continuous layer of reinforcement fibers 20 are fed into animpregnation zone 21 along a longitudinal path 22. Two resin films 23,24, without toughening particles, each supported by a release paper, areunwound from supply rollers 25, 26 and pressed onto the top and bottomsurfaces, respectively, of the fiber layer 20 via the aid ofpressure/consolidating rollers 27, 28, 29, 30 as the fiber layer 20moves through the pressure nips formed by the pressure rollers 27-30.Pressure from the pressure rollers 27-30 causes the resin films 23, 24to impregnate the fiber layer 20, resulting in a resin-impregnated fiberlayer 31. The release papers P1 and P2, which support resin films 23 and24, respectively, are then peeled off from the surface of theimpregnated fiber layer 31 after it passes through the second pressurenip between rollers 29 and 30. Next, two additional resin films 32, 33containing toughening particles are unwound from supply rollers 34, 35and are pressed onto the top and bottom surfaces, respectively, of theimpregnated fiber layer 31, via the aid of pressure/consolidatingrollers 36, 37, 38, 39, resulting in prepreg 40. Release paper P3, whichsupports resin 32, is peeled off from the prepreg 40 after it passesthrough the nip between rollers 38 and 39. As such, the resultingprepreg 40 is on a release paper and may be wound up at a downstreamlocation (not shown). The first two resin films 23, 24 may be pre-heatedin advance of and upstream to the pressure nips formed between opposingpressure rollers 27-30 in order to soften the resin films and tofacilitate the impregnation process. However, heating duringimpregnation is not sufficient to cure the resin matrix.

The four-film process described above is different from a two-filmprocess, which is more typical in the industry for forming a prepreg. Inthe two-film process, only two films of resin matrix are applied toopposite sides of a layer of reinforcing fibers using heat and pressureto thereby impregnate the fibers. When the resin matrix containstoughening particles that are larger than the interstices or gapsbetween adjacent fibers, the particles are filtered out by the fibersduring impregnation, and thus, remain on the outside of the fiber layer.

FIG. 3A illustrates another prepreg forming system, which is similar tothat shown in FIG. 2 except that a bead of resin 41 is formed betweentwo release papers P4, P5 ahead of the first nip formed betweenpressure/consolidating rollers 44, 45 to impregnate the layer 42 ofreinforcement fibers, and the resin content is controlled by changingthe gap between rollers 44, 45 to form a resin-impregnated fiber layer48. The bead of resin 41 is an accumulation of excess resin purposelybuilt up ahead of the first nip formed between rollers 44, 45. Anexploded view of the resin bead 41 is shown in FIG. 3B. As thereinforcement fiber layer passes through the excess resin it becomescoated with the resin itself. A resin which does not contain particlesis used in this step of the process. In one embodiment, the resin bead41 may be created by allowing some resin to be spread on one of therelease papers before it arrives at the first nip. The release papersP4, P5 are then peeled off from the resin-impregnated fiber layer 48after it passes through the second pressure nip formed by rollers 46 and47. Subsequently, two additional resin films 49, 50 containingtoughening particles are then pressed onto the resin-impregnated fiberlayer 48 to form a prepreg as described above in relation to FIG. 2.

Solubility

Determining whether certain particles are insoluble or soluble relatesto the solubility in a particular resin system in which they reside. Theresin system may include one or more thermoset resins, curing agentsand/or catalysts, and minor amounts of optional additives for modifyingthe properties of the uncured or cured resin matrix.

Hot stage microscopy can be used to determine if a particle isinsoluble, partially soluble, or swellable in a resin matrix. First, asample of dry polymeric particles (i.e., not combined with a resin) ismeasured to determine the average particle size and volume. Second, asample of particles is dispersed in the desired resin matrix viamechanical mixing. Third, a sample of the resulting mixture is placed ona microscope slide which is then placed in a hot stage setup under amicroscope. Then, the sample is heated to the desired cure temperature,and any change in size, volume or shape of the particles is observed andmeasured. All hot stage testing may be carried out at a particle loadingof 10 wt. % (weight percentage) of the resin matrix containing nocurative or catalyst.

Insoluble Toughening Particles

When the toughening particles are subjected to the above hot stagemicroscopy analysis and any change in diameter or volume of the particleis minimal, e.g., less than 5%, preferably less than 1%, as compared tothe original “dry” particles, then the particles are considered to beinsoluble. In some embodiments, insoluble toughening particles includeparticles that melt during the hot stage microscopy analysis but areincompatible with the resin matrix and therefore reform into discreteparticles upon cooling. For analytical purposes only, the particles mayflow during the hot stage microscopy analysis and the degree ofcrystallinity may also change.

For epoxy-based resin matrix, insoluble particles may include polymericparticles made of one or more polymers selected from: polyamideimide(PAI), polyamide (PA) polyetheretherketone (PEEK), polyetherketoneketone(PEKK), polyester, polypropylene, polyphenylene sulphide (PPS), liquidcrystal polymers (LCD).

In one embodiment, the insoluble particles are insoluble thermoplasticparticles that do not dissolve during the curing process and remainwithin the interlaminar regions of the cured composite material.Examples of suitable insoluble thermoplastic particles includepolyamideimide (PAI) particles and polyamide (PA) particles (e.g. nylonor polyphthalamide (PPA) particles), which are insoluble in epoxy resinsystem during the curing cycle thereof.

Certain grades of polyimide particles may be suitable as insolubletoughening particles. For example, polyimides prepared from benzophenonetetracarboxylic acid dianhydride (BTDA), 4,4′-methylenedianiline (MDA),and 2,4-toluenediamine (TDA), and having a non-phthalimide carboncontent which contains between 90 and 92 percent aromatic carbons (e.g.P84 available commercially from Lenzing AG).

Insoluble thermoplastic particles have been found to be effective asinterlaminar tougheners to avoid the loss of hot wet performance.Because these thermoplastic particles remain insoluble in a resin matrixeven after curing, they impart improved toughness, damage tolerance, hotwet performance, processing, micro-cracking resistance, and reducedsolvent sensitivity to the cured resin.

In addition to the above polymeric particles, inorganic particles formedof conductive materials (e.g., metal, graphite, carbon), ceramic,silica, may also be added as insoluble particles.

Partially Soluble and Swellable Toughening Particles

If the particle undergo partial dissolution and does not fully dissolvein a resin matrix upon thermal curing of the resin matrix, then theparticle is considered to be partially soluble. When such partiallysoluble particles are subject to the above hot stage microscopyanalysis, the change in diameter or volume of the particle is more than5% when compared with the original “dry” particles, but the particle isstill discernable as a discrete particle after curing and cooling. Asused herein, “dissolving” in a resin means forming a homogeneous phasewith the surrounding resin.

“Swellable” particles include particles that increase in the particlediameter or volume by more than 5% when subjected to the above hot stagemicroscopy analysis. The swelling is caused by the infusion of thesurrounding resin matrix into the outer surface of the particle.

Partially soluble or swellable thermoplastic particles have been foundto impart good tensile strength properties to a composite material.Certain engineered cross-linked thermoplastic particles are particularlysuitable as interlaminar toughening particles. These cross-linkedthermoplastic particles may be considered as partially soluble, and atthe same time, swellable.

Engineered Cross-Linked Thermoplastic Particles

In one embodiment, the engineered cross-linked thermoplastic particle iscomposed of a cross-linking network created by crosslinking across-linkable thermoplastic polymer having one or more one or morereactive groups with a cross-linking agent that is chemically reactiveto the reactive groups, wherein the cross-linking agent directlycross-links the polymer chains to each other via the reactive groups.The reactive groups may be end groups or pendant groups on the polymerbackbone. The direct cross-linking reaction of this embodiment may bedescribed as “tying-up” the polymer molecules via direct cross-linkingof the polymer chains using one or more reactive groups.

Cross-linked thermoplastic particles may be produced by an emulsionprocess, which includes dissolving the thermoplastic polymer, thecrosslinking agent, and a catalyst into a common solvent, which isimmiscible with water. An emulsion is then created in water by using anon-ionic surfactant, whereby emulsified particles are formed. Theemulsified particles are subsequently dried and cured so that thepolymeric chains become chemically cross-linked. The reacting conditionsand the type and level of crosslinking agent will determine the finalproperties of the particles. Reacting conditions such as temperatureresult in greater crosslinking. Crosslinking agents with greaterfunctionality will affect the extent of the crosslinking of thethermoplastic particles. Other crosslinking agents with relatively lowerfunctionality will crosslink to a lesser extent. The crosslinking agentconcentration will also be directly proportional to the extent ofcrosslinking.

Examples of suitable thermoplastic polymers that are susceptible tocross-linking include, but are not limited to, those selected from:polyethersulfones (PES) with hydroxyl end groups; polyetherimides (PEI)with hydroxyl end groups, amine groups or anhydride end groups;polyphenyleneoxides (PPO) or polyphenylene ether (PPE) with hydroxyl endgroups; polyaryletherketones (PAEK), including polyetheretherketone(PEEK), polyetherketoneketone (PEKK), with fluoro- or hydroxyl endgroups; or any engineering thermoplastic polymers with reactive endgroups or main chain functional groups. Specific examples ofcross-linkable thermoplastic polymers includes PES with hydroxyl endgroups, PES-PEES copolymer with amine end groups, PEI with amine endgroups, PPE with hydroxyl end groups.

Depending on the chemical nature of the thermoplastic polymer's endgroups/functionalities, an appropriate multifunctional crosslinkingagent with multiple reactive sites may be selected. Examples of suchcrosslinking agents are: alkylated melamine derivatives (e.g. Cymel303), acid chlorides (e.g. 1,3,5benzenetricarbonyl trichloride),multi-functional epoxies (e.g. Araldite MY0501, MY721), carboxylic acids(e.g. 1,2,4,5-benzenetetra-carboxylic acid). Polyunsaturatedthermoplastic polymers may also be easily cross-linked using radicaladdition using heat, UV or other radiation curing technique.

In another embodiment, the invention provides an engineered particlecomposed of an inter-penetrating polymer network (IPN), which is made upof thermoplastic polymer chains intertwined with an independentcross-linking network. The IPN is created by reacting one or morecompounds (e.g. cross-linkable monomers) having one or more reactivegroups with a cross-linking agent that is chemically reactive to thereactive groups in the presence of a thermoplastic polymer. The reaction(which occurs under certain cross-linking or curing conditions) causesthe compounds to become cross-linked via the reactive groups, therebyforming an independent cross-linking network. As such, the thermoplasticpolymer chains are intertwined with the independent cross-linkingnetwork at a molecular level to form an IPN. This approach may bedescribed as “tying-up” the thermoplastic polymer chains via theformation of a separate and independent cross-linking network, therebycreating an inter-penetrating network. Thus, in this embodiment, thethermoplastic polymer does not need to have reactive groups thereon.

As an example, an IPN may be created by: (i) forming an emulsioncontaining a thermoplastic polymer, a multifunctional epoxy resin and anamine curing agent capable of cross-linking the epoxy resin; (ii)removing the solvent from the emulsion and collecting the condensate,which is in the form of solid particles; (iii) drying the particlesfollowed by curing (e.g. by heating) so that the epoxy resin becomescross-linked. As a result of curing, the cross-linked epoxy forms an IPNwith the thermoplastic polymer.

The cross-linked thermoplastic particles described herein arethermodynamically compatible with a thermoset resin matrix, such as anepoxy-based matrix, and they are chemically cross-linked in order toprevent their total dissolution in the resin during curing of the resinmatrix.

The cross-linked thermoplastic particles described herein also form a“gradient interface” with the surrounding resin matrix in which theyreside. The term “gradient interface” as used herein refers to thegradual and strong interface between each of the particles and thesurrounding resin matrix. A gradient interface is achieved by usingengineered cross-linked thermoplastic particles that arethermodynamically compatible with the thermoset resin, e.g. epoxy. Theconcentration of thermoplastic polymer in the core of a cross-linkedthermoplastic particle is greatest at the center and gradually decreasestowards the outer surface of the particle as the resin matrix enters theparticle from the outer surface and moves towards the core. This gradualdecrease in the thermoplastic concentration from the core to the outersurface of the thermoplastic particle forms the gradient interfacebetween each of the thermoplastic particles and the surrounding resinmatrix. Thus, there is no sharp delineation or transition between thethermosetting resin and the thermoplastic particle. If a sharpdelineation or transition was present, the interface between thethermoplastic and the thermosetting resin would be much weaker in acomposite material in comparison to a composite material containing agradient interface. As such, the cross-linked thermoplastic particlesmay also be considered “swellable” because the resin matrix can diffuseinto the particles when the particles are mixed into the resin matrix,thereby resulting in an increase in the particle size. However, thecross-linked particles will remain as discrete and discernable particlesafter curing of the resin matrix.

“Discrete particle” as used herein refers to a particle which isdiscernible in a resin matrix, and which may be detected by usingScanning Electron Microscopy (SEM), Optical Microscopy, or DifferentialInterference Contrast microscopy (DIC).

The benefit of the cross-linked thermoplastic particles is the abilityto achieve locally high concentration of thermoplastic in theinterlaminar region without facing the risk of obtaining a phaseinversion. The thermoplastic content in the interlaminar region is knownto increase the toughness of the material. However, when largequantities of linear compatible thermoplastic are blended with ordissolved into a thermoset resin, the thermoplastic is known to phaseseparate in an inverted manner during the cure of the thermoset resin,also known as reaction induced phase separation, leading to athermoplastic continuous phase with inclusions of thermoset polymer.This phase inversion, in turn, is severely detrimental to the propertiesof the composite, primarily for temperature resistance and solventresistance.

Other examples of partially soluble and/or swellable thermoplasticparticles that are suitable for interlaminar toughening include certaingrades of polyimide particles. Thermoplastic polyimides useful for thepurposes discussed herein may swell or be partially soluble in the resinsystem at least during the curing cycle, but they must also resistdissolving to such an extent that they remain as discrete particlesafter curing. Not all polyimides perform equally for such application.Polyimides which have solubilities so great that they dissolvecompletely during the preparation of the resin matrix or during theprepreg fabrication process are not suitable.

Polyimides based on benzophenone tetracarboxylic acid dianhydride (BTDA)and 5(6)-amino-1-(4′-aminophenyl)-1,3,3-trimethylindane (AATI), andcontain only approximately 81 percent aromatic, 40 non-phthalimidecarbons, would be useful for the purposes discussed herein. Likewise,those based on mixtures of AATI and MDA or TDA would be expected towork, so long as the aromatic, non-phthalimide carbon content is lessthan 90%. Other polyimides expected to be useful are those where thediamine is based in whole or in part on2,2,4-trimethylhexane-1,6-diamine. Polyimides based on BTDA and AATI arealso suitable. Such polyimides are available commercially under thetrademark MATRIMID® 5218 from the Ciba-Geigy Corporation.

Additional examples of swellable particles include functionalized rubberparticles. Functionalized rubber particles are formed of functionalizedelastomers, which may include diene and olefin rubbers having, or havebeen modified to include carboxyl, carboxamide, anhydride, epoxy, oramine functionality. These rubber particles are further characterized asbeing partially crosslinked, such that they will exhibit sufficientintegrity to resist being solubilized appreciably at temperatures thatwill normally be encountered during the fabricating and curing of thecomposite in which they are incorporated.

In general, the insoluble and partially soluble/swellable particles mayhave particle sizes or diameters in the range of 5-70 μm. The particlesmay be regular or irregular in shape, and may take the form of sphericalparticles, milled particles, pellets, etc.

In a composite, the total amount of toughening particles (insoluble andpartially soluble/swellable particles) may constitute about 2% to 30% ofthe weight of resin matrix. Preferably, the content of tougheningparticles is within the range of 5% to 20% by weight. The optimum amountwill depend on the inherent toughness of the resin matrix, the toughnessof the particles, as well as other factors.

Resin Matrix

The resin matrix (or resin system), in which the toughening particlesare dispersed, refers to a curable resin formulation and may contain oneor more thermoset resins, which include, but are not limited to, epoxyresins, bismaleimide, vinyl ester resins, cyanate ester resins,isocyanate modified epoxy resins, phenolic resins, benzoxazine,formaldehyde condensate resins (such as with urea, melamine or phenol),polyesters, acrylics, and combinations thereof. In one embodiment, theresin matrix is an epoxy-based thermoset formulation which contains oneor more multifunctional epoxy resins as the main polymeric component.

Suitable epoxy resins include polyglycidyl derivatives of aromaticdiamine, aromatic mono primary amines, aminophenols, polyhydric phenols,polyhydric alcohols, polycarboxylic acids. Examples of suitable epoxyresins include polyglycidyl ethers of the bisphenols such as bisphenolA, bisphenol F, bisphenol S and bisphenol K; and polyglycidyl ethers ofcresol and phenol based novolacs.

Specific examples are tetraglycidyl derivatives of4,4′-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether,triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, bromobisphenol Fdiglycidyl ether, tetraglycidyl derivatives of diaminodiphenylmethane,trihydroxyphenyl methane triglycidyl ether, polyglycidylether ofphenol-formaldehyde novolac, polyglycidylether of o-cresol novolac ortetraglycidyl ether of tetraphenylethane.

Commercially available epoxy resins suitable for use in the resin matrixinclude N,N,N′,N′-tetraglycidyl diamino diphenylmethane (e.g. MY 9663,MY 720, and MY 721 from Huntsman);N,N,N′,N′-tetraglycidyl-bis(4-aminophenyl)-1,4-diiso-propylbenzene (e.g.EPON 1071 from Momentive);N,N,N′,N′-tetraclycidyl-bis(4-amino-3,5-dimethylphenyl)-1,4-diisopropylbenzene,(e.g. EPON 1072 fromMomentive); triglycidyl ethers of p-aminophenol(e.g. MY 0510 from Hunstman); triglycidyl ethers of m-aminophenol (e.g.MY 0610 from Hunstman); diglycidyl ethers of bisphenol A based materials(e.g. Tactix 123 from Huntsman); 2,2-bis(4,4′-dihydroxy phenyl) propane(e.g. DER 661 from Dow, EPON 828 from Momentive), glycidyl ethers ofphenol Novolac resins (e.g. DEN 431, DEN 438 from Dow);di-cyclopentadiene-based epoxy novolac (e.g. Tactix 556 from Huntsman);diglycidyl 1,2-phthalate (e.g. GLY CEL A-100); diglycidyl derivative ofBisphenol F (e.g. PY 306 from Huntsman). Other epoxy resins includecycloaliphatics such as 3′,4′-epoxycyclohexyl-3,4-epoxycyclohexanecarboxylate (e.g. CY 179 from Huntsman).

Generally, the resin matrix contains one or more thermoset resins incombination with additives such as curing agents, catalysts,co-monomers, rheology control agents, tackifiers, rheology modifiers,inorganic or organic fillers, soluble thermoplastic or elastomerictoughening agents, stabilizers, inhibitors, pigments/dyes, flameretardants, reactive diluents, and other additives well known to thoseskilled in the art for modifying the properties of the resin matrixbefore or after curing.

The addition of curing agent(s) and/or catalyst(s) may increase the curerate and/or reduce the cure temperatures of the resin matrix. The curingagent for thermoset resins is suitably selected from known curingagents, for example, aromatic or aliphatic amines, or guanidinederivatives. An aromatic amine curing agent is preferred, preferably anaromatic amine having at least two amino groups per molecule, andparticularly preferable are diaminodiphenyl sulphones, for instancewhere the amino groups are in the meta- or in the para-positions withrespect to the sulphone group. Particular examples are 3,3′- and4-,4′-diaminodiphenylsulphone (DDS); methylenedianiline;bis(4-amino-3,5-dimethylphenyl)-1,4-diisopropylbenzene;bis(4-aminophenyl)-1,4-diisopropylbenzene;4,4′methylenebis-(2,6-diethyl)-aniline (MDEA from Lonza);4,4′methylenebis-(3-chloro, 2,6-diethyl)-aniline (MCDEA from Lonza);4,4′methylenebis-(2,6-diisopropyl)-aniline (M-DIPA from Lonza);3,5-diethyl toluene-2,4/2,6-diamine (D-ETDA 80 from Lonza);4,4′methylenebis-(2-isopropyl-6-methyl)-aniline (M-MIPA from Lonza);4-chlorophenyl-N,N-dimethyl-urea (e.g. Monuron);3,4-dichlorophenyl-N,N-dimethyl-urea (e.g. Diuron™) and dicyanodiamide(e.g. Amicure™ CG 1200 from Pacific Anchor Chemical).

Suitable curing agents also include anhydrides, particularlypolycarboxylic anhydrides, such as nadic anhydride, methylnadicanhydride, phthalic anhydride, tetrahydrophthalic anhydride,hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride,endomethylenetetrahydrophtalic anhydride, and trimellitic anhydride.

Reinforcement Fibers

For fabricating high-performance composite materials and prepregs, thereinforcing fibres for the purposes described herein may becharacterized in general terms as having a high tensile strength (TS)(e.g. greater than 3500 MPa) and a high tensile modulus (TM) (e.g.greater than 230 GPa). Fibres useful for these purposes include carbonor graphite fibres, glass fibres and fibres formed of silicon carbide,alumina, titania, boron and the like, as well as fibres formed fromorganic polymers such as for example polyolefins, poly(benzothiazole),poly(benzimidazole), polyarylates, poly(benzoxazole), aromaticpolyamides, polyaryl ethers and the like, and may include mixtureshaving two or more such fibres. Preferably, the fibres are selected fromglass fibres, carbon fibres and aromatic polyamide fibres, such as thefibres sold by the DuPont Company under the trade name Kevlar.Furthermore, the reinforcement fibres to be impregnated with the resinmatrix may be in the form of a sheet of continuous, unidirectional ormultidirectional fibers, or as woven or nonwoven fabrics.

Composite Parts and Prepreq Layup

Multiple layers of prepreg plies may be laid up in a stackingarrangement to form a structural composite part that has a laminatedstructure, followed by curing. In certain embodiments, the prepreg plieswithin the layup may be positioned in a selected orientation withrespect to one another. For example, a prepreg layup may include prepregplies having unidirectional fiber architectures, with the fibersoriented at various angles, 0°, 45°, 90° etc., with respect to thelargest dimension of the layup, such as the length. It may be furtherunderstood that, in certain embodiments, the prepregs having anycombination of fiber architectures, such as unidirectional andmulti-dimensional, may be combined to form the prepreg layup. Theprepreg layup may be formed on a shaping tool to obtain a desiredthree-dimensional configuration. Curing of the prepreg layup usuallyoccurs under heat and pressure.

EXAMPLES

The following examples serve to illustrate the product and process ofthe present disclosure.

Example 1 Two-Film Process

A resin matrix was prepared based on the formulation shown in Table 1.

TABLE 1 Components Amounts [wt %] Araldite PY306 23.6 Araldite MY051023.6 PES 18.9 4,4′ DDS 23.9 Aromatic nylon (dry) 5.0 Cross-linkedPES-PEES particles 5.0

The resin matrix was then filmed onto a support paper to form a resinfilm having a film aerial weight (FAW) of 50 gsm.

Toho Tenax IMS65 carbon fibres were spread in a prepreg machine to anaerial weight of 194 gsm. Two resin films were then pressed onto eachopposing side of the spread fibres to obtain a prepreg with thefollowing characteristics:

-   FAW=194 gsm-   Resin Content=34%

Sheets cut from the above prepreg were laid up according to EN 2565 toform laminates. The laminates were then cured at 180° C. for 2 hoursusing a cure ramp rate of 2° C./min to reach the curing temperature.FIG. 4 shows an optical microscopy image of the cured laminate (incross-sectional view).

Example 2 Four-Film Process

Two different resin matrices were formed based on the formulations shownin Tables 2 and 3.

TABLE 2 Components [U-Film] Amounts (wt %) Araldite PY 306 26.2 AralditeMY 0510 26.2 PES 21.0 4,4′ DDS 26.6

TABLE 3 Components [P-Film] Amounts (wt %) Araldite PY 306 21.0 AralditeMY 0510 21.0 PES 16.8 4′4′ DDS 21.2 Aromatic nylon 10.0 Cross-linkedPES-PEES particles 10.0In the above Tables:

-   Araldite PY 306=Bi-functional epoxy based on bisphenol F-   Araldite MY 0510=Triglycidyl ether of p-aminophenol

The resin matrix based on the formulation of Table 2 was then filmed toan aerial weight of 25 gsm on a support paper to obtain a resin filmlabeled as “U-Film”. The resin matrix based on the formulation of Table3 was then filmed to an aerial weight of 25 gsm on a support paper toobtain a resin film labeled as “P-film”. Toho Tenax IMS65 carbon fibreswere spread in a prepreg machine to an aerial weight of 194 gsm. Usingthe four-film process described above, two U-films were pressed ontoopposite sides of the spread fibres to obtain a resin-impregnatedprepreg with the following characteristics:

-   FAW=194 gsm-   Resin Content=20%

Two P-films were then pressed onto opposite sides of the prepregobtained in the previous step to obtain a final prepreg with thefollowing characteristics:

-   FAW=194 gsm-   Resin Content=34%

Sheets cut from the above prepreg were laid up according to EN 2565 toform laminates. The laminates were then cured at 180° C. for 2 h using acure ramp rate of 2° C./min to reach the curing temperature.

FIG. 5 shows an optical microscopy image (X10 magnification) of thecured laminate, in cross-sectional view, fabricated from the four-filmprocess.

FIGS. 6A and 6B are ×20 magnified views of the interlaminar regions ofthe cured laminates shown in FIG. 4 and FIG. 5, respectively.

As can be seen from FIGS. 4, 5, 6A, and 6B, the laminate structureobtained by the four-film process has a much more uniform interlaminarregion as compared to that obtained by the two-film process. Moreover,while for the laminate produced by the two-film process, a significantamount of particles appears to have migrated away from the interlaminarregion and are embedded in within the fiber tows (FIGS. 4 and 6A), thisdoes not appear to be the case for the laminate produced by thefour-film process (FIGS. 5 and 6B) as most of the particles are confinedto the interlaminar regions.

Mechanical Test Results

The mechanical properties of the cured laminates fabricated according toExamples 1 and 2 were measured according to the test methods disclosedin Table 4. The test results are also shown in Table 4.

TABLE 4 Example1 Example2 Properties Units Test method Act. Norm. Act.Norm. G2c-Crack 1 J/m2 Modified 971 NA 1776 NA G2c Average of 3 cracksJ/m2 prEN6034 (*) 753 NA 1097 NA CAI-3S CAI MPa ASTM 308 300 307 300 26JCPT mm D7136/37 0.184 0.186 0.185 0.186 dent depth mm 0.156 0.163 (witin30 min) CAI-3S CSAI MPa 288 280 289 286 30.5J CPT mm 0.184 0.186 0.1870.186 dent depth ksi 0.180 0.183 (witin 30 min) (*) Coupon width was12.7 mm. Coupons pre-cracked in G_(IIc) configuration rather tha G_(Ic)configuration as specified in prEN6034.

The data summarized in Table 4 clearly show an increase in Mode IIinterlaminar toughness (G_(IIc)) associated with the laminate having amore uniform interlaminar region.

1-10. (canceled)
 11. A method for fabricating a curable prepreg plycomprising: moving a continuous layer of reinforcement fibers along alongitudinal path which comprises a plurality of pressure nips, eachpressure nip being formed by two opposing consolidating rollers;bringing two inner resin films into contact with the moving layer ofreinforcement fibers before the layer of reinforcement fibers passesthrough a first pressure nip such that one of the inner resin films isin contact with a top surface of the layer of reinforcement fibers andthe other inner resin film is in contact with the bottom surface of thelayer of reinforcement fibers; moving with the layer of reinforcementfibers with the inner resin films thereon through the first pressure nipand a second pressure nip to press the inner resin films onto the topand bottom surfaces of the layer of reinforcement fibers so as to form aresin-impregnated fiber layer having a top surface and a bottom surface;bringing two outer resin films into contact with the resin-impregnatedfiber layer before the resin-impregnated fiber layer passes through athird pressure nip such that one of the outer resin films is in contactwith the top surface of the resin-impregnated fiber layer and the otherouter resin film is in contact with the bottom surface of theresin-impregnated fiber layer; and moving the resin-impregnated fiberlayer with the outer resin films thereon through the third pressure nipand a fourth pressure nip to press the outer resin films onto the topand bottom surfaces of the resin-impregnated fiber layer, wherein theinner resin films are formed from a first curable resin matrixcomprising at least one thermoset resin, but is void of any tougheningparticles, the outer resin films are formed from a second curable resinmatrix comprising at least one thermoset resin, and a mixture of twodifferent types of toughening particles: (i) insoluble tougheningparticles, and (ii) partially soluble or swellable toughening particles,and said insoluble toughening particles are insoluble in the secondresin matrix upon curing of the prepreg ply, and said partially solubleor swellable toughening particles are partially soluble or swellable inthe second resin matrix upon curing of the prepreg ply, but remain asdiscreet particles after curing.
 12. A method for fabricating acomposite structure comprising: forming a plurality of prepreg plies,each prepreg ply being formed by the method of claim 11; laying up theprepreg plies in a stacking arrangement to form a laminate structure;and curing the laminate structure, wherein the insoluble tougheningparticles are insoluble in the second resin matrix upon curing, and thepartially soluble or swellable toughening particles are partiallysoluble or swellable in the second resin matrix upon curing, but remainas discreet particles after curing.
 13. The method of claim 11, whereinthe second resin matrix comprises at least one epoxy resin, and saidinsoluble toughening particles are thermoplastic particles that areinsoluble in epoxy resin upon curing of the epoxy resin.
 14. The methodof claim 11, wherein the second resin matrix comprises partially solubletoughening particles, which are thermoplastic particles that decrease involume by more than 5% upon curing of the prepreg ply, but remain asdiscreet particles after curing.
 15. The method of claim 11, wherein thesecond resin matrix comprises swellable toughening particles, which arethermoplastic particles that increase in volume by more than 5% prior toor during curing of the prepreg ply.
 16. The method of claim 11, whereinthe second resin matrix comprises a combination of insolublethermoplastic particles and swellable, crosslinked thermoplasticparticles, said crosslinked thermoplastic particles comprising one of:(a) a crosslinking network created by crosslinking a cross-linkablethermoplastic polymer having at least one reactive group with across-linking agent that is chemically reactive to the reactive group,and (b) an inter-penetrating polymer network (IPN) comprisingthermoplastic polymer chains intertwined with a separate cross-linkingnetwork, said IPN being created by reacting at least one compound havingone or more reactive groups with a cross-linking agent that ischemically reactive to the one or more reactive groups, in the presenceof a thermoplastic polymer.
 17. A method for fabricating a curableprepreg ply comprising: moving a continuous layer of reinforcementfibers along a longitudinal path which comprises a plurality of pressurenips, each pressure nip being formed by two opposing consolidatingrollers; applying a first curable resin in the form of a resin beadahead of a first pressure nip as the layer of reinforcement fiberspasses through the first pressure nip, wherein the resin content iscontrolled by altering the gap between the consolidating rollers of thefirst pressure nip to form a resin-impregnated fiber layer; moving theresin-impregnated fiber layer through a second pressure nip; pressing anouter resin film onto the top surface of the resin-impregnated fiberlayer and an outer resin film onto the bottom surface of theresin-impregnated fiber layer by moving the the resin-impregnated fiberlayer with the outer resin films thereon through pressure nips locateddownstream from the second pressure nip, wherein the outer resin filmsare formed from a second curable resin, which comprises at least onethermoset resin, and a mixture of two different types of tougheningparticles: (i) insoluble toughening particles, and (ii) partiallysoluble or swellable toughening particles, said first curable resincomprises one or more thermoset resins, but is void of the sameinsoluble and partially soluble or swellable toughening particles, andsaid insoluble toughening particles are insoluble in the second resinupon curing of the prepreg ply, and said partially soluble or swellabletoughening particles are partially soluble or swellable in the secondresin upon curing of the prepreg ply, but remain as discreet particlesafter curing.
 18. The method of claim 17, wherein the second resinmatrix comprises at least one epoxy resin, and said insoluble tougheningparticles are thermoplastic particles that are insoluble in epoxy resinupon curing of the epoxy resin.
 19. The method of claim 17, wherein thesecond resin matrix comprises partially soluble toughening particles,which are thermoplastic particles that decrease in volume by more than5% upon curing of the prepreg ply, but remain as discreet particlesafter curing.
 20. The method of claim 17, wherein the second resinmatrix comprises swellable toughening particles, which are thermoplasticparticles that increase in volume by more than 5% prior to or duringcuring of the prepreg ply.
 21. The method of claim 17, wherein thesecond resin matrix comprises a combination of insoluble thermoplasticparticles and swellable, crosslinked thermoplastic particles, saidcrosslinked thermoplastic particles comprising one of: (a) acrosslinking network created by crosslinking a cross-linkablethermoplastic polymer having at least one reactive group with across-linking agent that is chemically reactive to the reactive group,and (b) an inter-penetrating polymer network (IPN) comprisingthermoplastic polymer chains intertwined with a separate cross-linkingnetwork, said IPN being created by reacting at least one compound havingone or more reactive groups with a cross-linking agent that ischemically reactive to the one or more reactive groups, in the presenceof a thermoplastic polymer.